Page 279 - Industrial Power Engineering and Applications Handbook
P. 279

11/258  Industrial Power Engineering and Applications Handbook

                              X  (minus)+
             0   -0.1   -0.2  -0.3  -0.4   -0.5  -0.6   -0.7   -0.8   -0.9  -1.0
           0.6                                              1.1
           0.5                                              1 .o

                                                           0.9
        t  0.4                                                          W2
                                                                           =
        a 0.3                                              0.8       x= - ratio of watt meter readings
        ul                                                              WI
        8                                                                   of like polarity.
           0.2                                              0.7     -x=-!% w,  = ratio of watt meter readings of
                                                                              unlike polarity.
           0.1                                              0.6
                              x (plus) -
            0                                               0.5
             0   0.1   0.2   0.3   0.4   0.5  0.6  0.7   0.8  0.9   1.0

                              Figure 11.3  Power Factor versus ratio of  watt meter readings




          Dynamometer                                 Pull-out torque
          Pony brake
          Rope and pulley                             The motor  should be mounted  with  a suitable loading
          Calibrated machine.                         arrangement.  The rated voltage,  at the rated frequency,
                                                      is then  applied  to  the  motor  terminals  at  the  no-load
        Readings of voltage, current and speed should be taken.   condition. The load on the motor may then be gradually
        The torque value is obtained directly by the dynamometer,   increased,  and the maximum  load at which the motor
        pony brake and rope and pulley methods and indirectly   stalls noted. The torque determined at this point is the
        by  the calibrated  machine  method.  Speed-torque  and   pull-out torque.
        speed-current tests must be conducted at a rated voltage
        or as near to this as practical. When it is necessary to
        establish values of current and torque at the rated voltage,   Note
                                                       1  The motor should be immediately disconnected from the supply
        based on tests made at reduced voltage, the current may   when it stalls.
        increase by  a ratio higher  than  the first  power  of  the   2  The motor should not be kept in the locked rotor condition for
        voltage and the torque by a ratio higher than the square   more than a few seconds to avoid damage to the windings.
        of the voltage due to possible saturation of flux leakage
        paths. See  also Figure 27.2(b) and note under serial number   11.4.6 Vibration measurement test
        9 Section 7.19 for more clarity. This relationship varies
        with the design and, as a first approximation, is sometimes   The vibration limits have been classified into two groups:
        taken as the  current varying  directly  with  voltage and
        torque with the square of the voltage.         1  For shaft heights 56 mm and above, IS0 2373 (IEC
          It is therefore necessary to take precautions during the   60034- 14) has prescribed three categories of vibration
        test to avoid a excessive temperature rise and  consequent   levels in terms of vibration velocity,  one for normal
        damage to the windings. For wound rotor motors, speed-   use, N, and the other two for precision  applications,
        torque  and  speed-current tests  may  be taken between   i.e.  reduced  level  R  and  special-purpose  S. When
        synchronous speed and the speed at which the maximum   required other than normal, these must be specified
        torque occurs.                                   by  the  user  to  the  manufacturer.  Machines  with  a
                                                         higher degree of balance should be used only when
        Pull-up torque                                   this  is  essential.  Such  machines  may  be far  too
                                                         expensive  to  produce,  and  sometimes  not  com-
        The motor  should be mounted  with  a suitable loading   mensurate with the application.
        arrangement and the rotor fully locked. The rated voltage   2  For shaft heights more than 400 mm, IEC 60034-14
        at the rated frequency will then be applied to the motor   prescribes  the  vibration  level,  in  terms  of  double
        terminals in the locked rotor condition. The loading on   amplitude vibration, which can also be derived from
        the motor will then be reduced slowly so that the motor   the velocity of vibration, using the following formula:
        can start and pick up speed. The value of pull-up torque
        at which the  rotor picks  up  speed and  attains  speed   a = 0.45 x %!!&
        corresponding to pull-out torque condition must be noted.   f
   274   275   276   277   278   279   280   281   282   283   284